The present invention relates to a thermal printing system which forms picture images on a printing sheet by using a thermal printing head, and more particularly relates to a thermal printing system of such a type which can selectively either form direct images which are the usual way around or can form reversed or mirror type images.
Thermally perforable stencils for use in thermal printing are known in various forms. One such type, an example of which is schematically shown in perspective in FIG. 7 of the appended drawings (the characters inscribed on this stencil should be ignored with respect to the immediately following descriptions), consists of a sheet F of thermoplastic or heat resolving film made, for example, of polyvinyl chloride copolymer, polyethylene telefterate, polypropylene or the like adhered against a sheet P of porous laminar material such as Japanese paper which can transmit ink through itself while supporting the thermally perforable thermoplastic material sheet F in an overall manner. A compound stencil sheet of this type can be thermally perforated in a desired perforation pattern by application of heat to parts of it, arranged in said pattern, on its side on which is situated the thermally perforable thermoplastic sheet F; when this happens, these parts of the sheet F of thermoplastic material melt and curl up and effectively disappear, leaving only the backing sheet P of porous material present in these heated up portions of the compound stencil sheet, supporting the non perforated portions of the thermoplastic sheet F therearound. Thus the desired pattern is reproduced on the compound stencil sheet in the form of local perforations. Subsequently this perforated stencil sheet can be mounted in a stencil printing device, and then printing ink can be applied to its side on which is situated the porous laminar material sheet P, while its other side on which is situated the thermally perforable thermoplastic sheet F is pressed against paper to be printed on: when this is done, the printing ink passes through the thermally perforated parts of the stencil sheet in which only the porous laminar sheet P remains and inks the printing paper, while on the other hand the non perforated parts of the stencil sheet in which still the thermoplastic sheet F is locally present intercept the passage of the printing ink.
Now, one per se well known way for preparing and perforating such a thermally perforable stencil sheet has been to press the stencil sheet against an original on which the pattern to be reproduced is inscribed in a dark pattern against a light ground, and then to flash infrared radiation against the original through the stencil sheet: thus the dark portions of the original become heated up and transfer their heat to the corresponding portions of the stencil sheet, thus bringing them above their critical temperature at which the thermoplastic sheet F thereof melts and causing them to be perforated. In such a utilization mode, as will be readily apparent, when later printing is performed as indicated above with the side of the perforated stencil sheet on which is situated the thermally perforable thermoplastic sheet F being contacted against the printing paper and with the other side of the perforated stencil sheet charged with printing ink, then the image reproduced on the printing paper is an exact copy of the original and is the same way around as the original, so no problem is caused.
However, another manner for using such a thermally perforable stencil sheet could be to directly perforate it by direct application of heat from a heat pattern generating unit such as a thermal printer head of a thermal printing system. In fact, thermal printers are per se well known, in which a printer head comprises a plurality of electrically operated thermal dot generating elements. Such printers are used nowadays in typewriters, word processors, and facsimile machines. If such a printer is being used for printing on thermally markable paper, no printing ribbon is used but instead the head is pressed against such paper with the dot generating elements opposing the paper, and then, as the head is moved along the paper and the dot generating elements are rapidly selectively heated up by selective supply of electrical energy to them in appropriate patterns and timings, patterns and characters are printed on the paper in a dot matrix fashion. (The printer may also be used for printing on ordinary paper, in which case a thermal printing ribbon is interposed between the printing head and the paper). As a matter of course, such printers as available currently are so structured, and the patterns and timings for heating of their dot generating elements are so set up, that they produce characters the right way around on the printing paper, i.e. they do not produce mirror image characters but produce hard copy with the characters thereon the usual way round. Now, if such a thermally perforable stencil sheet as described above were to be directly inserted into such a thermal printer with the side of the stencil sheet on which is situated the thermally perforable thermoplastic material sheet F opposing the printing head, then by operation of the printer as described above the stencil sheet could be directly perforated. However, it will be clear based upon the above explanations that, if this was done and printing was performed from the stencil sheet which had been perforated as above in the manner detailed previously, the patterns produced on the final printing paper would be reversed from the first original patterns produced by the printing head on the stencil sheet, in other words would be the mirror images of the images that would have been produced on thermal paper if such had been fitted into the thermal printer rather than the thermally perforable stencil sheet. In particular, writing would not be readable but would be backwards. This fact has up till now been an obstacle to this type of direct perforation of a thermal stencil sheet. Nor can the problem be resolved by reversing the orientation in which the stencil sheet is fitted into the thermal printer, because if the side of the perforated stencil sheet on which is situated the porous laminar material sheet P is the one opposing the thermal printing head, the heating up of the thermal dot generating elements will not be effective for properly perforating the thermally perforable sheet F, because of the heat capacity of the porous laminar material sheet P interposed therebetween which prevents proper and accurate perforation.